The invention relates to apparatus for electrical discharge machining of a circumferential section from a bore in a metallic cylinder, and more particularly, to such apparatus for machining a circumferential section having an axial cross-section in the shape of an isosceles trapezoid from a rotor.
To ensure the continued safe operation of a turbine or generator rotor beyond its original design life, it is necessary to periodically evaluate the condition of the rotor. The condition of the rotor can be analyzed through various nondestructive and destructive techniques. It is preferred, where possible, to use nondestructive methods to evaluate the rotor so as not to further mechanically weaken the rotor through destructive procedures. However, data obtained through use of even the most advanced nondestructive techniques, such as the use of eddy currents, are difficult to quantify, and thus are not generally useful in determining the life expectancy of the rotor. Therefore, information necessary to make a reliable assessment of the life expectancy of the rotor must be obtained through destructive methods.
Normally, such destructive procedures require that samples be taken from all critical locations of the rotor, that is, those locations subject to particularly high operating stresses. Thus, because the rotor bore is one such critical location, the rotor bore must be examined in order to properly assess the condition of the rotor. However, because the rotor bore is difficult to access to obtain a sample thereform, it has been heretofore impossible to reliably evaluate the condition of the wall of the rotor bore.
Thus it is desired to develop apparatus that is capable of obtaining a sample section of the wall of the rotor bore that can be used to evaluate the condition of the wall of the rotor bore. It is further desired that the shape of the sample section be such that the continued operation of the rotor is not significantly adversely affected by the removal of the section.